A variety of image readers have been proposed heretofore. Such an image reader subjects image information from a document to line scanning in the horizontal direction, forms an image of this information on a line sensor such as a CCD serving as reading means, and reads the image information from the document by utilizing an output signal obtained from the CCD.
FIG. 7 is a diagram illustrating an example of the main components of an image reading apparatus according to the prior art. The image reading apparatus shown in FIG. 7 is capable of reading a document by two methods, namely a flow-scan reading method whereby image information is read from documents while the documents are transported and fed one after another, and a fixed reading method whereby the document is held fixed and illuminating and an image forming system is caused to scan the document to read the image information from the document.
As shown in FIG. 7, a first document 301, whose image information is to be read while the document is held fixed, is placed on a platen glass 302 provided at a location where the image information can be read by the fixed reading method. This apparatus includes a light source 303, a slit 304, first, second and third mirrors 305, 306 and 307, respectively, an image forming lens 308, and a line sensor 309 such as a CCD. A second document 310 has its image information read while it is transported by an automatic document feeder, which is not shown. These documents can be transported one after another. A document transport glass 311 is provided at a location where the image information can be read from the document by the flow-scan reading method. The second document 310 passes by the upper side of the document transport glass 311 and is transported by a roller 312.
The document reading operation performed by the flow-scan reading method is such that the image reader optical system, which includes the light source 303, slit 304, first, second and third mirrors 305, 306 and 307, respectively, image forming lens 308 and line sensor 309, is moved to a point underlying the document transport glass 311 before the second document 310 is read.
Next, second documents 310 are transported one after another from the automatic document feeder (not shown). When a second document 310 is transported by the roller 312 and passes by the upper side of the document transport glass 311, the second document 310 is illuminated by the light source 303, the light reflected from the illuminated second document 310 passes through the slit 304 and the document image is formed on the image forming surface of the line sensor 309 by the image forming lens 308 through the intermediary of the first, second and third mirrors 305, 306 and 307. Since the second documents 310 are transported successively and pass by the upper side of the document transport glass 311, a signal based upon the image information of each second document 310 in the main scanning direction thereof is obtained during transport in the sub-scanning direction, whereby the image information of the second document 310 is read.
Thus, by merely providing a document reading section at the forward end of the image reading apparatus and providing a simple automatic document feeder to feed documents, the documents (limited to documents in the form of sheets) are fed automatically to make possible the flow-scan reading of image information.
With the fixed reading method, the first document 301, which has been placed on the platen glass 302, is illuminated by the light source 303, light reflected from the first document 301 that passes through the slit 304 is acted upon by the image forming lens 308 so that an image is formed on the line sensor 309, and the light source 303, slit 304 and first, second and third mirrors 305, 306 and 307 are made to scan the first document 301 in the sub-scanning direction, thereby reading the image information from the first document 301.
By thus reading a document while the document is held fixed, it is possible to read image information from a document bound in book form or from an original that is difficult to feed using an automatic document feeder, such as a thick document or thin fabric.
In an image reading apparatus of the above-described kind, the second document 310 is transported from the automatic document feeder to a point above the document transport glass 311. In order to facilitate transport of the second document 310 and deal with the attachment of contaminants such as dust or the like, the surface of the document transport glass 311 is provided with an electrically conductive film (referred to also as an “EC coat”), such as a film of ITO (Indium Tin Oxide), or is treated to lower the frictional resistance thereof.
More specifically, providing the surface of the document transport glass 311 with an electrically conductive film allows the flow of an electric current, thereby preventing the build-up of electric charge between the second document 310 and the document transport glass 311 so as to enhance transportability of the second document 310. Further, treating the surface of the document transport glass 311 to reduce the frictional resistance thereof lowers the coefficient of friction between the second document 310 and the document transport glass 311, thereby stabilizing transport of the second document 310.
The treatments described above also are effective in preventing the attachment of dust to the document transport glass 311.
In addition, a synergistic effect is obtained by subjecting the surface of the document transport glass 311 to both the conductive-film treatment (the EC coat) and friction reducing treatment simultaneously.
However, when the surface of the document transport glass 311 is coated with the electrically conductive film or treated to lower frictional resistance, this causes a change in the spectral transmittance of the document transport glass 311.
FIG. 8 is a graph illustrating the spectral transmittance of ordinary glass whose surface has not been treated in any way and of treated glass whose surface has been subjected to the conductive-film and low-friction treatments.
FIG. 8 demonstrates that there is a difference between the spectral transmittance (the solid line in FIG. 8) of float plate glass (ordinary glass) of 4-mm thickness (one example) used in an ordinary image reading apparatus, and the spectral transmittance (dashed line in FIG. 8) of glass (treated glass) obtained by subjecting the surface of float plate glass to conductive-film and low-friction treatments.
In general, glass whose surface has been subjected to conductive-film and low-friction treatments exhibits a spectral transmittance lower than that of untreated glass, with the difference being particularly pronounced in the short wavelength region. Consequently, when the surface of the document transport glass 311 is provided with the electrically conductive film or is subjected to the low-friction treatment, the image information obtained by reading a document by the flow-scan reading method will differ from that obtained by reading the document by the fixed reading method.
Further, since the above-mentioned disparity is particularly conspicuous on the side of short wavelengths, the effect of treating the glass is particularly great when image information is read from a color document. This means that color image information obtained by the flow-scan reading method will differ from that obtained by the fixed reading method.
To better understand the above-mentioned phenomenon, consider a case where ordinary glass is used as the platen glass for the fixed reading method and glass, whose surface has been subjected to the conductive-film treatment and low-friction treatment, is used as the document transport glass for the flow-scan reading method. Owing to the difference between the spectral transmittances shown in FIG. 8 in such case, first the light that illuminates the document does not arrive at the document in the same way, then the light reflected from the document is influenced again because it passes through the glass a second time. In other words, the difference in the spectral transmittance of the glass becomes a factor twice, i.e., once for the illuminating light and once for the reflected light, with regard to color information obtained by the flow-scan reading method and color information obtained by the fixed reading method.
In order to solve this problem, subjecting the surface of the platen glass for the fixed reading method to the conductive-film and low-friction treatments has been contemplated. However, the platen glass for reading a fixed document has a size larger than that of the document, as depicted in FIG. 7. Applying an electrically conductive film and friction reducing treatment to the surface of a platen glass that is so much larger than the document would result in much higher cost.